If you didn't read up on the game ahead of time, then you should hear about it from me now. In Total War: Arena you don't play a large army, just 3 units, just like 9 other players do the same. (Thus 30 units meet 30 units in battle) Your units are always assigned to a specific faction, even if all factions in the game are thrown together. You can currently choose from: Romans, barbarians, Greeks, Carthaginians, Chinese and gladiators.

In addition to the three units, you choose a commander, which grants your troops 3 abilities each (more on that later).